1. Field of the Invention
The present invention relates generally to a state determining apparatus for an exhaust gas purifier which purifies exhaust gases exhausted from an internal combustion engine, and more particularly, to a state determining apparatus for an exhaust gas purifier which purifies exhaust gases by adsorbing hydrocarbons contained in the exhaust gases by an adsorbent.
2. Description of the Prior Art
One type of internal combustion engine is provided with an adsorbent disposed in its exhaust system for adsorbing hydrocarbons in exhaust gases upon starting the engine. The adsorbent has, for example, zeolite on its surface, such that hydrocarbons within exhaust gases enter into pores of the zeolite and are adsorbed by the adsorbent when the hydrocarbons pass through the adsorbent. As the adsorbent is heated to a predetermined temperature or higher (for example, 100-250xc2x0C.) by exhaust gases, the adsorbent desorbs once adsorbed hydrocarbons which are recirculated to the internal combustion engine through an EGR pipe and the like. While the adsorption and desorption of hydrocarbons are repeated in the adsorbent in the foregoing manner, a long-term use of the adsorbent may lead to a gradually increasing amount of residual hydrocarbons which could not be desorbed, and broken pores of the adsorbent. As a result, the adsorbent is deteriorated, causing gradually degraded capabilities of adsorbing hydrocarbons in the adsorbent. When the internal combustion engine is started in such a state, hydrocarbons not adsorbed by the adsorbent are emitted to the outside. Thus, a need exists for determining the state of the adsorbent, in particular, a deterioration thereof.
The applicant has proposed a deterioration determining apparatus for determining a deterioration of such an adsorbent, for example, in Laid-open Japanese Patent Application No. 2001-323811. This deterioration determining apparatus takes advantage of a proportional relationship found between the capabilities of the adsorbent to adsorb hydrocarbons and moisture, and detects the humidity of exhaust gases which have passed through the adsorbent by a humidity sensor to determine degraded capabilities of the adsorbent to adsorb hydrocarbons and moisture, i.e., the deterioration of the adsorbent. More specifically, the deterioration determining apparatus sets a predetermined time required for a detected humidity of the humidity sensor to rise by a predetermined value associated with a gradual rise in humidity of exhaust gases which pass through the adsorbent, while moisture in the exhaust gases is adsorbed by the adsorbent, after the start of the engine, in accordance with the humidity at the start and the like, with reference to a normal adsorbent which has not been deteriorated, and measures a time actually taken by the detected humidity to rise by the predetermined value. Then, when the measured time is shorter than the predetermined time, the adsorbent is determined to be deteriorated from the fact that the rising speed of the detected humidity is higher or the detected humidity begins to rise at an earlier timing than when a normal adsorbent is used.
However, the deterioration determining apparatus described above could fail to ensure a sufficient determination accuracy for the deterioration of the adsorbent, because the humidity detected by the humidity sensor rises at a different rising rate or at a different timing depending on a temperature state of the exhaust system at the start of the internal combustion engine.
More specifically, for example, when an internal combustion engine is cold started, heat generated by exhaust gases is taken away by an exhaust system which has substantially the same temperature as the temperature in a starting environment (outside air temperature), so that the temperature of the exhaust gases is lower at a location more downstream of the exhaust system. Then, as the temperature decreases to the dew point (for example, 50-60xc2x0 C.), moisture in exhaust gases begins condensing and attaches on the inner surface of an exhaust pipe and the like, so that the moisture in exhaust gases decreases more at a location more downstream in the exhaust system. Such condensation occurs more, at an earlier time, and at a more upstream location in the exhaust system as the exhaust system is at a lower temperature upon starting. For this reason, the detected humidity tends to present a lower rising rate because the adsorbent is supplied with exhaust gas with less moisture, i.e., with a lower humidity when the condensation occurs at a location upstream of the adsorbent. This tendency becomes stronger as the exhaust system is at lower temperatures when the internal combustion engine is started.
Also, as described above, the adsorbent desorbs hydrocarbons when it is heated to a predetermined temperature or higher, and tends to have a higher performance of adsorbing hydrocarbons as the temperature is lower at the temperature at which the desorption begins (for example, 50xc2x0 C.) or lower. This tendency is likewise found in the moisture adsorbing performance as well as in the hydrocarbon adsorbing performance. Therefore, when the combustion engine is cold started, a detected humidity rises at a later timing because a larger amount of moisture in exhaust gases is adsorbed in the adsorbent as the temperature of the adsorbent having substantially the same temperature as the temperature in the exhaust system is lower.
As described above, while the detected humidity rises at a different rate and at a different timing depending on a temperature state of the exhaust system upon start of the internal combustion engine, the aforementioned deterioration determining apparatus merely sets a time elapsed from the start as a parameter for determining a deterioration of the adsorbent, so that it only provides an insufficient accuracy for determining the deterioration, and therefore leaves a room for improvement in this respect.
The present invention has been made to solve the problems as mentioned above, and it is an object of the invention to provide a state determining apparatus for an exhaust gas purifier which is capable of accurately determining the state of the exhaust gas purifier, which includes an adsorbent for adsorbing hydrocarbons, including a deterioration of the adsorbent, in accordance with a temperature state of an exhaust system in an internal combustion engine.
To achieve the above object, the present invention provides a state determining apparatus for an exhaust gas purifier arranged in an exhaust system of an internal combustion engine for determining a state of the exhaust gas purifier including an adsorbent capable of adsorbing hydrocarbons and moisture in exhaust gases. The state determining apparatus is characterized by comprising a humidity sensor arranged near the adsorbent in the exhaust system for detecting a humidity in an exhaust pipe of the exhaust system; temperature state detecting means for detecting a temperature state in the exhaust system; and adsorbent state determining means for determining a state of the adsorbent in accordance with the humidity in the exhaust pipe detected by the humidity sensor and the temperature state in the exhaust system detected by the temperature state detecting means.
According to this state determining apparatus for an exhaust gas purifier, the humidity sensor arranged near the adsorbent in the exhaust system detects a humidity in an exhaust pipe of the exhaust system, while the temperature state detecting means detects a temperature state in the exhaust system. Then, the adsorbent state determining means determines the state of the adsorbent in accordance with the results of determinations. Since the abilities of the adsorbent to adsorb hydrocarbons and moisture is in a proportional relationship, the humidity detected by the humidity sensor has a high correlation to actually adsorbed hydrocarbons in the adsorbent. Also, as mentioned above, the humidity detected by the humidity sensor rises at a different rate and at a different timing depending on the temperature state in the exhaust system. Therefore, when the temperature state in the exhaust system is used as a parameter in addition to the humidity within the exhaust pipe for making the state determination, the state of the adsorbent can be accurately determined, including adsorption and desorption of hydrocarbons in the adsorbent, deterioration of the adsorbent, and the like, while reflecting the actual temperature of exhaust gases to the determination result.
Preferably, the state determining apparatus for an exhaust gas purifier further comprises calory calculating means for calculating calory supplied from the internal combustion engine to the exhaust system after the internal combustion engine is started, wherein the adsorbent state determining means further determines the state of the adsorbent further in accordance with the calory calculated by the calory calculating means.
According to this preferred embodiment of the state determining apparatus, the calory calculating means calculates the calory supplied from the internal combustion engine to the exhaust system after the internal combustion engine is started, and the adsorbent state determining means further determines the state of the adsorbent further in accordance with the calory calculated by the calory calculating means. It is therefore possible to more accurately determine the state of the adsorbent while additionally reflecting the states of the exhaust system and a temperature change (rise) of the adsorbent after the start to the determination result. Preferably, the state determining apparatus for an exhaust gas purifier further comprises threshold determining means for determining a threshold based on the temperature state in the exhaust system detected at the time the internal combustion engine is started, wherein the calory calculating means includes accumulated fuel injection amount calculating means for calculating an accumulated fuel amount supplied to the internal combustion engine from a start thereof as the calory, and the adsorbent state determining means determines the state of the adsorbent based on a result of comparison between the accumulated fuel amount from the start of the internal combustion engine and the threshold when a changing amount of the value detected by the humidity sensor after the internal combustion engine is started exceeds a predetermined value set therefor.
According to this preferred embodiment of the state determining apparatus, the state of the adsorbent can be determined at an appropriate timing at which a changing amount of the value detected by the humidity sensor after the start of the internal combustion engine exceeds the predetermined value set therefor, i.e., at which the humidity in the exhaust pipe sufficiently increases (rises) as the adsorption to the adsorbent is gradually saturated. Also, the state of the adsorbent is determined based on the result of a comparison of the accumulated fuel amount supplied to the internal combustion engine from the start to the time at which the humidity in the exhaust pipe sufficiently increases with the threshold determined by the threshold determining means. This threshold reflects the temperature state in the exhaust system at the start of the internal combustion engine, while the accumulated fuel amount indicates the calory given to the exhaust system after the start. Therefore, by determining the state of the adsorbent based on the result of the comparison of the accumulated fuel amount with the threshold, the state of the adsorbent can be more accurately determined while well reflecting actual temperatures of the exhaust system and adsorbent at the time the internal combustion engine is started, and after the start, to the determination result. Also, since the fuel amount is a known control parameter for the internal combustion engine, the calory given to the exhaust system can be readily calculated by simply accumulating the fuel amount from the start of the internal combustion engine.
Preferably, the state determining apparatus for an exhaust gas purifier further comprises ambient temperature detecting means for detecting an ambient temperature around the humidity sensor; and relative humidity calculating means for calculating a relative humidity of exhaust gases from an output of the humidity sensor in accordance with the detected ambient temperature.
According to this preferred embodiment of the state determining apparatus, since the humidity of exhaust gases is calculated from the output of the humidity sensor in accordance with the ambient temperature around the humidity sensor, it is possible to appropriately find the relative humidity compensated for temperature. In addition, the state of the adsorbent can be appropriately determined in accordance with the relative humidity found in this manner.
Preferably, in the state determining apparatus for an exhaust gas purifier, the humidity sensor is arranged at a location downstream of the adsorbent in the exhaust system.
According to this preferred embodiment of the state determining apparatus, since the humidity sensor is arranged at a location downstream of the adsorbent in the exhaust system, the humidity sensor can detect the humidity of exhaust gases which have passed through the adsorbent after the internal combustion engine had been started. Thus, the humidity reflecting an adsorbed state of hydrocarbons in the adsorbent can be detected during the operation of the internal combustion engine, to accurately determine the state of the adsorbent.
Preferably, in the state determining apparatus for an exhaust gas purifier, the adsorbent state determining means determines the state of the adsorbent after a stop of the internal combustion engine.
According to this preferred embodiment of the state determining apparatus, since the state of the adsorbent is determined after the internal combustion engine is stopped, the state of the adsorbent can be determined without the need for monitoring the humidity in the exhaust pipe by the humidity sensor at all times, unlike the state determination for the adsorbent made during the operation of the internal combustion engine. The state of the adsorbent can be determined in this manner for the following reasons. As the internal combustion engine is stopped, the heated adsorbent is gradually cooled down, and accordingly adsorbs surrounding moisture. Then, as the adsorbent incrementally adsorbs the moisture to saturation, the humidity around the adsorbent remains substantially constant, i.e., in the steady-state. The humidity in the steady-state reflects the state of the adsorbent, particularly a deterioration degree. Specifically, a larger deterioration degree of the adsorbent indicates that the adsorbent has a lower moisture adsorbing performance, so that the adsorbent does not adsorb much moisture around it. As the result, the humidity around the adsorbent tends to be higher as compared with that around a normal adsorbent. It is therefore possible to determine the state of the adsorbent by detecting the humidity around the adsorbent after the internal combustion engine is stopped. In addition, since the state determination only requires the detection of the humidity in the exhaust pipe which remains in the steady-state, a humidity sensor is not required to have a high responsibility, so that a reasonable humidity sensor can be used, thereby reducing the cost of the overall apparatus.
Preferably, in the state determining apparatus for an exhaust gas purifier, the adsorbent state determining means determines the state of the adsorbent within a predetermined period after the internal combustion engine is stopped.
According to this preferred embodiment of the state determining apparatus, the state of the adsorbent can be accurately determined by executing the state determination within the predetermined period after the internal combustion engine is stopped, i.e., within a period in which the state determination can be appropriately made for the adsorbent. As described above, when the internal combustion engine is stopped, the adsorbent adsorbs surrounding moisture as the exhaust system is cooled down, bringing the humidity around the adsorbent into the steady-state. When a long time elapses after the internal combustion engine is stopped, the humidity within the exhaust pipe, which has been in the steady-state, gradually converges to the external humidity because the exhaust system is in communication with the outside. Therefore, the state of the adsorbent can be appropriately and accurately determined by executing the state determination while the humidity within the exhaust pipe remains in the steady-state until it begins converging to the external humidity.
Preferably, in the state determining apparatus for an exhaust gas purifier, the humidity sensor is arranged at a location upstream of the adsorbent in the exhaust system.
According to this preferred embodiment of the state determining apparatus, the humidity sensor is spaced from the most downstream end of the exhaust system in communication with the outside by a longer distance than when the humidity sensor is arranged at a location downstream of the adsorbent, so that the humidity sensor can be prevented from being adversely affected by disturbance such as the influence of external air when the internal combustion engine is stopped, for example, a gas exchange between the atmosphere within the exhaust pipe near the humidity sensor and the external air. In this manner, the state determination can be appropriately and accurately made for the adsorbent.
Preferably, the state determining apparatus for an exhaust gas purifier further comprises operating condition detecting means for detecting whether or not the internal combustion engine is operated in a predetermined operating condition before the internal combustion engine is stopped, wherein the adsorbent state determining means determines the state of the adsorbent when the operating condition detecting means detects that the internal combustion engine is operated in the predetermined operating condition before the internal combustion engine is stopped.
According to this preferred embodiment of the state determining apparatus, the state determination is made for the adsorbent when the internal combustion engine is operated in the predetermined operating condition before the internal combustion engine is stopped. Generally, the amount of moisture contained in exhaust gases depends on the operating condition of the internal combustion engine. Therefore, the state of the adsorbent can be accurately determined after the internal combustion engine is stopped by defining the predetermined operating condition as an operating condition in which exhaust gases contain an amount of moisture suitable for determining the state of the adsorbent.
Preferably, in the state determining apparatus for an exhaust gas purifier, the predetermined operating condition of the internal combustion engine is a condition in which an air/fuel mixture supplied to the internal combustion engine is at an air/fuel ratio near the stoichiometric air/fuel ratio during the operation of the internal combustion engine.
According to this preferred embodiment of the state determining apparatus, the deterioration determination is made for the adsorbent after the internal combustion engine is stopped during an operation near the stoichiometric air/fuel ratio. When the internal combustion engine is operated near the stoichiometric air/fuel ratio, exhaust gases contain a relatively large amount of moisture without large variations, so that the ambient humidity around the humidity sensor after the engine is stopped is also relatively high without large variations, suitable for making the deterioration determination for the adsorbent. Thus, the deterioration determination made in such a condition can provide a more accurate determination as to whether the adsorbent is deteriorated.
Preferably, in the state determining apparatus for an exhaust gas purifier, the temperature state of the exhaust system is a temperature of a cooling water upon start of the internal combustion engine.
According to this preferred embodiment of the state determining apparatus, the temperature of cooling water in the internal combustion engine upon starting can be properly used as a parameter indicative of the temperature state in the exhaust system. Also, since the internal combustion engine is typically provided with a water temperature sensor for detecting the temperature of cooling water, the existing water temperature sensor may be used for detecting the operating condition, thereby realizing the temperature state detecting means at a low cost.
Preferably, in the state determining apparatus for an exhaust gas purifier, the adsorbent state determining means includes adsorbent deterioration determining means for determining a deterioration of the adsorbent as the state of the adsorbent.
As described above, a deteriorated adsorbent suffers from lower abilities to adsorb moisture as well as hydrocarbons, so that the adsorbent can be determined in regard to deterioration by detecting the humidity around the adsorbent. Therefore, according to the preferred embodiment of the state determining apparatus, a deterioration of the adsorbent can be accurately determined by the state determining technique of the present invention so far described, while satisfactorily reflecting the temperature state in the exhaust system of the internal combustion engine to the result of determination.
Preferably, in the state determining apparatus for an exhaust gas purifier, the adsorbent comprises zeolite.
According to this preferred embodiment of the state determining apparatus, the zeolite adsorbs moisture as well as hydrocarbons, and there is a high correlation between the abilities of the zeolite to adsorb both components, so that the advantages and effects so far described can be well provided by applying the present invention. The zeolite can implement an adsorbent which excels in heat resistance and is less susceptible to deterioration, as compared with, for example, silica gel, active carbons or the like when they are used as the adsorbent.